The present invention relates to electroencephalography and more particularly to a method and system for comparing the signals from selected pairs of electrodes detecting a subject's brain waves.
It is well known that the brain may produce weak electrical signals both spontaneously and in response to external stimuli. These brain waves may be detected by an electrode contact removably attached to the surface of the head. The electrodes are generally connected by a conductive paste and the system of attaching the electrodes generally follows the International Association 10-20 system which specifies the anatomical points to which the various electrodes should be connected. Those electrical signals generally, at the present time, are amplified and displayed by movements of a pen on a paper in an EEG (electroencephalograph) apparatus. The brain waves (cortical responses) which are present during test periods may be the result of spontaneous brain activity during these periods or may result from external stimuli, i.e., an evoked response, for example, from a flashing light or an audio click.
Neurologists, trained by a long period of study, may monitor the EEG recording and study the paper strip chart and recognize some form of brain injury from those charts. Generally such neurologists operate on a rather subjective or pattern recognition approach and may be able to associate specific wave patterns on the chart with particular types of injury or locations of injury. It has long been recognized that there are not a sufficient number of such skilled neurologists to conduct many of the EEG tests which would be useful in both clinical and screening situations. For example, the EEG analysis is not part of a routine medical examination or even those medical examinations which are used to screen selected personnel.
It has also been suggested that the analysis of the subject's brain waves may be performed by special analog computers, special digital computers or general digital computers which are programmed with a suitable program.
One of the problems that has arisen in the obtaining of brain wave information is that the amount of data which would be useful to be obtained may require an extended testing period. For example, it may be desirable to expose the subject under test to an extensive series of stimuli in order to obtain his responses to the stimuli. Many of the subjects, particularly if they are children or aged, may tire easily, become restless under such extended testing conditions, or change their responses with the familiarization of repetition. Consequently, it is desirable that the test procedures take up as little of the subject's time as possible. In addition, the time of the technician running the test and the time of the test apparatus may both be relatively costly. Consequently, it is again desirable that the test procedure should provide the maximum possible amount of information and at the same time not consume a large amount of the time of the technician or of the testing apparatus.
It has been suggested in the past that, in addition to the brain wave information derived from each individual electrode, valuable information concerning the subject's brain waves may be obtained by comparing the brain waves obtained from one electrode with the brain waves simultaneously obtained from a different electrode.
In the article, "An On-Line Transformation of EEG Scalp Potentials Into Orthogonal Source Derivations," Electroencephalography and Clinical Neurophysiology, 1975, it is explained that the scalp may be shown to be made up of 19 concentrated potential fields, each one representing actual currents in the corresponding area of the skull. An electrode will detect the orthogonal activity and the signals may be mapped accordingly. That article is of interest only in showing the use of other than direct measurements.
In the present inventor's prior U.S. Pat. No. 2,696,808 entitled "Method and System for Brain Wave Analysis" there is disclosed a special instrument which considers the two signals from electrodes placed in bilateral symmetric positions on the subject's head. The instrument amplifies the two signals, cross-correlates them, and provides at its output a cross-correlation coefficient of the two signals.
In addition, it has been suggested that a comparison between the signals from two electrodes may be obtained by connecting the electrodes as the inputs to a difference amplifier, as is described in U.S. Pat. No. 4,037,586. A difference amplifier produces at its output a signal representing the difference between the two input signals and consequently the difference amplifier would produce a signal representing the difference in the brain wave signals at the two selected electrodes. A major difficulty with this approach, however, is that it may unduly prolong the testing period. For example, if one wishes to obtain a recording of 19 electrodes recorded monopolarly (one at a time) and, in addition, if one wishes to obtain a recording of the difference signals between any selected 9 pairs (bipolar derivations) of tests, the testing period would be doubled for a specified series. If one then wished to obtain another set of comparisons or difference signals between selected electrodes, one would again be required to repeat the entire series of tests.